This invention relates to a process for making an elastically stretchable composite sheet and more particularly to such a composite sheet of comfortable touch being suitable as an important component of disposable garments such as disposable diapers, sanitary napkins and disposable gowns used in medical site.
Japanese Patent Publication No. 1995-37703 discloses a process for making a nonwoven elastic sheet comprising the steps of a continuously feeding elastic web with a tension in a machine direction, placing a fibrous web on the upper surface of the elastic web, bonding them together by heat- or ultrasonic-sealing and relaxing the elastic web to contract so that the fibrous web may form gathers. The nonwoven elastic sheet thus obtained has an elasticity generated by the presence of the elastic web and a comfortably soft touch so that such nonwoven elastic sheet may be suitably used as cover material of disposable diapers or sanitary napkins.
However, the process of prior art is necessarily accompanied with a problem that a basis weight of the fibrous web being fed inevitably increases as the elastic web contracts after the fibrous web has been sealed with the elastic web.
This invention aims to provide a process for making an elastically stretchable composite sheet enabling the fibrous web in the finished composite sheet to maintain substantially the same basis weight as that of the fibrous web being fed and thereby to solve the problem in the process of prior art.
According to this invention, a process for making a composite sheet comprising a step of bonding a first web made of thermoplastic synthetic fibers and having an inelastic stretchability in one direction a first to at least one surface of a second web made of thermoplastic synthetic fibers having an elastic stretchability at least in the one direction and thereby to obtain the composite sheet having an elastic stretchability in the one direction, wherein:
the first web is made of stretchable synthetic continuous fibers having a breaking extension at least of 70% while the second web has its breaking extension higher than that of the first web and the first and second webs are bonded in accordance with the steps of:
a. continuously feeding the first web in the one direction;
b. continuously feeding the second web in the one direction so as to be placed upon the first web;
c. bonding the first and second webs together intermittently at least along the one direction rather than in a direction being orthogonal to the one direction;
d. stretching the first and second webs bonded together at least in the one direction rather than the direction being orthogonal to the one direction within a critical elasticity of the second web and a critical breaking extension of the first web; and
e. elastically relating the stretched first and second webs to contract them and thereby to obtain the composite sheet.
The composite sheet obtained by the process according to this invention is easily stretchable and offers a comfortable touch, so that the composite sheet is suitable as cloth and/or elastic member in disposable garments such as disposable pants or disposable gowns used in medical site.
The process according to this invention for making the elastically stretchable composite sheet enables the fibrous web in the composite sheet to have a basis weight lower than that in the conventional composite sheet because the stretchable fibrous web is laminated in its unstretched condition with the elastically stretchable web.
With the composite sheet of this invention, the component fibers of the fibrous web are stretched under a plastic deformation and have their diameter correspondingly reduced as the composite sheet is stretched once in the course of the process. At the same time, undesirable bonding and/or entangling among the component fibers themselves in the fibrous web and undesirable bonding between the fibrous web and the elastically stretchable web are loosened. Therefore, a relatively small force required to stretch the elastically stretchable web is sufficient as an initial force required to stretch the composite sheet so that the composite sheet may be easily stretched and offer a comfortable soft touch.